Currently, transparent electrodes have found applications to various displays, various solar batteries, and touch panels. To fabricate these transparent electrodes, a method has been widely used in which indium-tin oxide (ITO) is sputtered to form a film on a glass substrate, a plastic substrate or the like.
The use of this method allows formation, on the substrate, of a transparent conductive film having a surface resistance of several dozen to several hundred Ω/□, thus exhibiting superior conductivity. Nevertheless, the method poses the problem that it needs to use considerably expensive sputtering equipment, and also has the drawback that it finds difficulty in using substrates which are less resistant to heat because a substrate used in the method has to be exposed to heating during film formation.
Alternatively, a method has been proposed in which a transparent conductive film-forming coating liquid containing microparticles of noble metals and the like dispersed in a solvent is coated on a substrate by spin coating or the like, followed by drying, and thereafter, a transparent coat layer-forming coating liquid comprising silica sol is coated on the resultant dry coat, followed by drying and subsequent baking at a temperature in the vicinity of 200° C., thereby forming a transparent conductive film of a double-layer type (see Japanese Patent Laid-Open Publications No. 10-110123, No. 10-142401, No. 10-182191, No. 11-329071, No. 2000-124662 and No. 2000-196287).
However, such a method has to form a double-layered coated film by coating and drying a transparent conductive film-forming coating liquid and a transparent coat layer-forming coating liquid, respectively, and hence involves tedious process steps. Another problem is that it is difficult to ensure electrical connection with respect to the resulting transparent conductive film because a transparent coat layer of a relatively electrically insulative character is present on the film. Hence, this method is still unsatisfactory for fabricating a transparent electrode.
To cope with these problems, from some consideration of the above-noted conventional methods, a method may be regarded as feasible in which a transparent conductive film-forming coating liquid containing microparticles of noble metals and the like dispersed in a solvent is coated on a substrate, followed by drying, and the resultant dry coat is then baked to obtain a transparent conductive film of a single-layer type with no need for coating a transparent coat layer-forming coating liquid. The film formed by this method is problematic in that it is extremely poor in film strength, for example, to such an extent that it is simply releasable upon finger rubbing on its surface.
Moreover, the above-cited Japanese Patent Laid-Open Publications No. 11-329071 and No. 2000-124662 describe that film strength (scratch strength) can be improved by adding, to a transparent conductive film-forming coating liquid containing microparticles of noble metals and the like dispersed in a solvent, silica microparticles (colloidal silica) having an average particle diameter of 100 nm or below and set within the range of 1 to 60% by weight (preferably 20 to 40% by weight) based on the weight of the microparticles of noble metals and the like.
However, in the case of a transparent conductive film obtained by use of the above transparent conductive film-forming coating liquid alone, but by omission of a transparent coat layer-forming coating liquid, the film strength is still far from satisfactory. The film cannot simply be used as such in single-layer form. This is presumably because the conventional methods described in Japanese Patent Laid-Open Publications No. 11-329071 and No. 2000-124662 are contrived to use, as a binder component, colloidal silica having a considerably larger particle diameter (generally 5 to 20 nm in average particle diameter) than that of silica sol used in common coating liquids.
On the other hand, as a replacement for the above-noted conventional method in which both a transparent conductive film-forming coating liquid and a transparent coat layer-forming coating liquid are used, a method has been proposed in which a transparent conductive single-layer film is obtained by the use of a transparent conductive film-forming coating liquid containing silica sol or the like added as a binder component (see Japanese Patent Laid-Open Publications No. 09-115438 and No. 10-1777).
In the methods described in Japanese Patent Laid-Open Publications No. 09-115438 and No. 10-1777, however, a transparent conductive film-forming coating liquid is used which contains microparticles of noble metals and the like held monodisperse. In such an instance, in a step of drying the coating liquid, the binder component tends to fill in between the microparticles of noble metals and the like, obstructing microparticle-to-microparticle contact of the microparticles, so that the binder component needs to be used in controlled limited amounts.
Hence, the resulting transparent conductive film has a low strength corresponding to a pencil hardness of 3H or so. The film still causes insufficient strength.
That is, a transparent conductive film having low resistance and high transmittance can be obtained only when microparticles of noble metals and the like mutually interconnect in a step of drying a transparent conductive film-forming coating liquid and eventually provide a grown network structure. The methods described in Japanese Patent Laid-Open Publications No. 09-115438 and No. 10-1777 present the problem that a balance is difficult to establish between the improvement of a film strength made by adding a binder component and the provision of a network structure derived by using the above microparticles.
The present invention has been made with attention focused on the above-discussed problems. Objects of this invention are to provide a transparent conductive single-layer film formed on a substrate by spin coating or the like and having excellent conductivity, high transmittance and great strength, and a transparent conductive film-forming coating liquid for use in forming such a transparent conductive single-layer film.
Other objects of the invention are to provide a transparent conductive layered structure provided with such a transparent conductive single-layer film, and a display device fabricated by incorporating such a layered structure therein.